If one just wants an eBook, then besides the vast price difference, the Kindle does have a few advantages over the iPad. Some people get a headache when looking at a backlit screen over a long period and the Kindle is not backlit. The iPad requires Wi-Fi or a cellular plan to download books, but the Kindle operates via cellular without paying for a data plan. The Kindle weighs less than half that of the iPad.
Of course for other purposes, the iPad does have its own advantages, such as apps, colour screen, video playback, etc.